The memory in the 4850 is Memory Type GDDR3 my 4870 is GDDR5 Im not that up to date on the difference, would I be taking a step back or adding performance? ( CrossFire is all new to me )
AFAIK, there isn't much of a performance boost between DDR3 and 5 (correct me if wrong) but if your doing CrossFire with a HD 4850 GDDR3 with your 4870 you should notice about 40 - 60% performance boost in gaming regardless of the memory type, memory type doesnt matter when Cross-firing.